Hearing protection device

ABSTRACT

A device (1) for insertion into an ear canal of a mammalian subject. The device includes a body (2) having a sound path extending therethough and a tensioned membrane (10) in the sound path. The tensioned membrane has at least one corrugation (12). The device further includes an adjustable member (6) arranged to bear against the membrane to adjust the tension of the membrane and thereby to alter an acoustic response of the sound path. The adjustable member may include a compressible portion (8).

The present invention relates to ear protection used, for example, toreduce the intensity of sounds experienced by a user.

Exposure to high intensity noises can cause damage to a person'shearing. The damaging effects are increased when a person is frequentlyexposed to loud noises. In extreme cases, frequent exposure to loudnoises can cause noise-induced hearing loss. Therefore, in order toprotect a person's hearing it is necessary to reduce the effects of loudnoises. As a result of the increasing awareness of the damaging effectsof loud noises, for example from industrial sources, there are nowvarious industry requirements for personnel to use ear protection. Thereare many situations in which personnel may be exposed to loud noises,for example when operating loud machinery. A common form of noiseprotection widely used are earplugs; these reduce the intensity of thesound entering a person's ears and thus reduce the damaging effects ofhigh intensity noises.

There are two main types of earplugs that are commonly used: passiveearplugs and active earplugs. Passive earplugs attenuate the intensityof all levels of sound equally, i.e. they provide a uniform level ofattenuation, regardless of the intensity of sound present, for example,a reduction of 20 dB. Passive earplugs come in various forms including:foam, silicon, flanged and custom moulded earplugs. Passive earplugs aretypically inserted into a user's ear canal. Passive earplugs use thematerial of the earplug itself to attenuate the sound which passesthrough it. As the incident sound passes through the earplug, the soundis attenuated by the material of the earplug. Some sound will propagatethrough the earplug and pass out of the earplug into the air volume ofthe user's ear canal where it will be detected by the user. Theintensity of the sound will be reduced and thus the risk of damage tothe user's hearing may be reduced.

If they are inserted properly, the fixed level of attenuation providedby passive earplugs is relatively high. As a result, users of passiveearplugs typically have to periodically remove them in order to becommunicate orally with fellow workers.

The inconvenience associated with having to repeatedly remove andreplace the earplugs, depending on the noise levels, may lead to reducedcompliance with requirements to wear the earplugs in certain situations.

On the other hand, active earplugs often comprise a passive earplug usedin conjunction with a microphone on the external side of the earplug anda speaker on the internal side of the earplug. Active earplugs typicallylisten to the sound on the outside of the earplug, and then replay it toa user via the speaker at a reduced intensity. Active earplugs canemploy control circuitry to apply different levels of attenuation atdifferent times or different frequencies—known as ‘adaptiveattenuation’. Some other systems contain circuitry that detects theincident sound and produces an out-of-phase signal that destructivelyinterferes with the incident sound thus reducing the intensity of theincident sound as it propagates into the user's ear canal—this is oftenknown as ‘active noise cancelling’. One of the disadvantages of activeearplugs is that they are often relatively expensive due to theirelectrical components. Additionally, active earplugs often have arelatively high power consumption due to the need to constantly monitorand replay detected sound.

The present invention seeks to address or mitigate the problems outlinedabove and according to a first aspect there is provided a device, forinsertion into an ear canal of a mammalian subject, comprising:

-   -   a body, having a sound path extending therethough;    -   a tensioned membrane in the sound path comprising at least one        corrugation; and    -   an adjustable member arranged to bear against the membrane to        adjust the tension of the membrane and thereby to alter an        acoustic response of the sound path.

With the claimed arrangement of the tensioned membrane and theadjustable member, it is possible to control the sound passing throughthe sound path, e.g. the attenuation of the sound. By attenuating thesound, the device may reduce the intensity of sounds and therefore mayhelp to reduce the risk of damage to an individual's hearing. This isbecause when the device is inserted into the ear canal of an individual,the sound is attenuated by the device so that the sound which propagatesthrough the earplug and passes out of the earplug into the air volume ofthe user's ear canal (and reaching the user's eardrum) has a loweramplitude, than the sound which would propagate in the air volume of theuser's ear canal if the device was not inserted.

Attenuation of sound provided by the device may be adjusted by alteringthe tension of the membrane. The force with which the adjustable memberbears on the membrane may be adjusted to alter the tension of themembrane therefore changing the attenuation of sound by the device. Thisallows the level of attenuation of sound provided by the device to bevaried. For example, an individual using the device may reduce the levelof attenuation of sound provided by the device in order to communicateorally with fellow workers or an individual using the device mayincrease the level of attenuation of sound provided by the device uponentering an environment with higher amplitude noises.

It has further been appreciated by the Applicant that without thecorrugation referred to, the level of attenuation provided by the devicewould increase substantially when the adjustable member first contactsthe membrane compared to the level of attenuation provided when theadjustable member was not in contact with the adjustable membrane andwould increase rapidly thereafter, even with small movements. TheApplicant has appreciated that it would be desirable to be able to varythe level of attenuation provided more smoothly (e.g. with no largechanges in the level of attenuation provided) and more gradually, toallow finer control over the attenuation provided by the device.

The Applicant has further appreciated that such finer control over theattenuation provided by the device can be achieved by implementing atensioned membrane that includes at least one corrugation. When theadjustable member contacts the membrane, the corrugation is typically(at least partially) stretched due to the force the adjustable memberexerts on the membrane and a smaller increase in the tension of themembrane may be observed for a given applied force. Therefore, the levelof attenuation provided by the device may increase more gradually whenthe adjustable member contacts the membrane compared to a membranewithout any corrugation.

In a set of embodiments, the corrugation comprises a ridge. In apotentially overlapping set of embodiments, the corrugation comprises anindentation. Depending on the orientation of the membrane, an identicalcorrugation could be described as either a ridge or an indentation.Preferably the maximum height of the ridge above or depth of theindentation below the plane of the membrane is in the range of 0.02 mmto 2 mm e.g. 0.1 mm to 1 mm e.g. 0.3 mm.

In a set of embodiments, the corrugation is arranged on the adjustablemembrane such that the adjustable member does not contact thecorrugation. The adjustable member may deform the corrugation byapplying a force to another part of the membrane which is transmitted tothe corrugation.

In a set of embodiments, the adjustable member is arranged to contactthe membrane in the geometric centre of the membrane. In embodimentswherein the membrane is circular, the corrugation may be arranged on themembrane at a distance from the geometric centre of the membrane in therange of 30% to 90% of the radius of the membrane.

In a set of embodiments, the corrugation is circular and centred on thegeometric centre of the membrane.

In a set of embodiments, the adjustable membrane comprises a pluralityof corrugations. Different corrugations, or different subsets of thecorrugations within the plurality of corrugations, may have differentshapes and sizes. However, the plurality of corrugations may all havethe same shape.

The Applicant has envisaged a particular arrangement in which theplurality of corrugations comprises a subset of circular ridges andsubset of circular indentations, which may be centred on the geometriccentre of the membrane. The various circular indentations and ridgeswould have various diameters, and therefore could be arranged at variousdistances (i.e. locations) from the geometric centre of the adjustablemembrane.

Each circular ridge or indentation may be described as a wave. Thesewaves are preferably connected such that there are no abrupt changes inthe curvature of the membrane creating points of increased stress. Thismay be achieved by the circular indentations and ridges being arrangedto alternate, e.g. if the innermost circular corrugation is anindentation/negative wave, the second innermost circular corrugation isa ridge/positive wave, the third innermost circular corrugation is anindentation/negative wave etc. In a set of embodiments, the centre ofthe membrane is flat.

Whilst the membrane may comprise any number of waves, in a set ofembodiments the membrane comprises at least three waves. Preferably themembrane has three waves. In a particular set of embodiments, theinnermost wave and outmost waves are ‘positive’ waves extending higherthan the centre of the membrane (i.e. towards the direction of incidenceof sound in use), and the intervening wave is a ‘negative’ waveextending lower than the centre of the membrane (i.e. away from thedirection of incidence of sound during use). When sound is incident onthe membrane and the adjustable member contacts the membrane, thepositive waves may act as ‘hinges’ to generate movement of theintervening negative wave.

In embodiments in which the membrane comprises a plurality of waves, thedimensions of the waves may vary. For example, the height above or belowthe centre of the membrane, and/or the radial extent (width) of eachwave may vary. In some embodiments, the height and the radial extent(width) of a wave may depend on the distance (i.e. location) of the wavefrom the geometric centre of the membrane. However the height of eachwave and the width of each wave may be the same.

A wave may have a uniform cross-section section throughout thecircumference of the circle formed. However, this is not essential andindeed the Applicant has appreciated that it may be beneficial thatwaves do not have a uniform cross-section. More particularly in a set ofembodiments, the or each wave comprises a plurality of circumferentiallyspaced perturbations or ‘ripples’ thereon. Such perturbations may reducethe stress in the wave and allow the wave to move more freely whensubjected to vibrations, such that the level of attenuation can bechanged more smoothly and gradually to allow finer control over theattenuation provided by the device.

The number of perturbations on a given wave may be chosen to suit theapplication. In a set of embodiments, the number of perturbations on theor each wave is in the range 5 to 40, e.g. 10 to 30, e.g. 20.

The perturbations may be arranged in any suitable and desirable manner.Preferably the perturbations are evenly spaced around the circumferenceof the or each wave.

In a set of embodiment, the perturbations extend in an oppositedirection to the corresponding wave on which they are formed—i.e. theyappear as indentations on a wave when from viewed from above whichextend towards the tangential plane of the centre of the membrane.Alternatively however the perturbations may project proud of thecorresponding wave—i.e. extend further away from the tangential plane ofthe centre of the membrane.

Whilst the perturbations may extend radially, preferably theperturbations are non-radial. In such embodiments, the perturbations maybe angled with respect to the radius of the membrane. In a set ofembodiments, the angle between perturbations and the radius of themembrane is in the range of 5° to 85°, e.g. 20° to 70°, e.g. 45°.

In embodiments comprising a plurality of waves, one or more of the wavesmay comprise perturbations. It is preferable that every wave comprisesperturbations. Whilst perturbations may be arranged to extendcontinuously across more than one wave, preferably adjacent wavescomprises discrete sets of perturbations.

Different orientations of the perturbations with respect to the radiusof the membrane may be provided on different waves, e.g. negative andpositive waves may comprise perturbations with different orientations.Different orientations of perturbations may be provided by theperturbations extending in opposite directions with respect to theradius of the membrane. In a set of embodiments however allperturbations are oriented in the same direction relative to the radius.

The number of perturbations may vary on each wave. In a set ofembodiments however the number of perturbations on each wave is thesame. In such embodiments, the spacing of perturbations on largerdiameter waves will be less dense compared with smaller diameter waves.

Adjusting various attributes of the waves and perturbations results invariations in the stiffness of the membrane. Different waves andperturbation arrangements will result in a membrane with differentstresses and creeps. This may affect the sound attenuation provided bythe membrane and the quality of the attenuated sound. Different wave andperturbation arrangements may also affect the behaviour of the membranewhen the membrane is placed under different tensions by the adjustablemember. A particular arrangement of waves and perturbations maytherefore be selected to provide specific properties and performances ofthe membrane under particular tensions.

The device may comprise other features which help to provide increasedcontrol over the level of attenuation provided by the device. In a setof embodiments, the adjustable member comprises a compressible portion.The Applicant has appreciated that implementing an adjustable memberwith a compressible portion may be inventive in its own right withoutrequiring the device to have a membrane comprising at least onecorrugation. Therefore, when viewed from a second aspect the presentinvention provides a device, for insertion into an ear canal of amammalian subject, comprising:

-   -   a body, having a sound path extending therethough;    -   a tensioned membrane in the sound path; and    -   an adjustable member comprising a compressible portion and        arranged to bear against the adjustable membrane to adjust the        tension of the membrane and thereby to alter an acoustic        response of the sound path.

As with the first aspect of the invention, the arrangement of themembrane and the adjustable member allows for control of the soundpassing through the sound path, e.g. the attenuation of the sound.

When the adjustable member contacts the membrane, the compressibleportion of the adjustable member is compressed (and/or deformed),reducing the force exerted on the adjustable membrane by the adjustablemember and resulting in a smaller increase in the tension of themembrane. Without the compressible portion referred to, the level ofattenuation provided by the device would increase substantially when theadjustable member first contacts the membrane compared to the level ofattenuation provided when the adjustable member is not in contact withthe membrane and would increase rapidly thereafter, even with smallmovements. Therefore, as with the corrugation of the first aspect of theinvention, the level of attenuation provided by the device may increasemore smoothly and more gradually when the member contacts the membranecompared with a rigid member (i.e. with no compressible portion), toallow finer control over the attenuation provided by the device.

The compressible portion could be provided by any part of the adjustablemember, but in a set of embodiments the compressible portion of theadjustable member contacts the surface of the membrane. This may aid thestability of the adjustable member.

The adjustable member may be arranged to have any suitable and desirableshape. There may be embodiments in which the compressible portion (whichcomprises part of the adjustable member) does not have the same shape asthe remaining portion(s) of the adjustable member. For example, thecompressible portion may be a cylinder with a smaller diameter than theremaining portion(s) of the adjustable member.

The compressible portion of the adjustable member may be shaped to becompressible (e.g. comprising a spring) and/or the compressible portionmay be formed from a layer of inherently compressible material.Preferably, the compressible portion of the adjustable member is formedfrom an elastically compressible material. Whilst the Applicant hasappreciated that the adjustable member could be formed from any suitableand desirable compressible material, in a set of embodiments, of eitherthe first or the second aspect of the invention, the deformable portionis formed from thermoplastic elastomers or foam materials, e.g.polyurethane foam.

In a set of embodiments the adjustable member, or the rest of theadjustable member apart from the compressible portion, is formed from arigid material. Whilst the Applicant has appreciated that the adjustablemember could be formed from any suitable and desirable material,preferably the adjustable member is formed from a plastic. For example,the adjustable member is formed from a rigid plastic e.g.polycaprolactam (PA6), acrylonitrile butadiene styrene (ABS) orpolyoxymethylene (POM).

In a set of embodiments, of either the first or the second aspect of theinvention, the adjustable member comprises a central axis which isperpendicular to a plane or at least a central tangent plane of themembrane. In a set of embodiments, of either the first or second aspectof the invention, the adjustable member is rotationally symmetric abouta central axis. This may help to ensure a uniform force is exerted onthe membrane when the adjustable member is in contact with the membrane.

The adjustable member may be in contact with membrane throughout itstravel However, in a set of embodiments of either the first or thesecond aspect of the invention, the adjustable member has a positionwherein the adjustable member is not in contact with the membrane. Inthis position, the adjustable member does not exert a force on themembrane and therefore, the adjustable member does not alter the tensionof the membrane from a base value.

It will be appreciated by the skilled person that when the adjustablemember is in the non-contact position, the base level of attenuationprovided by the device is lower than the level of attenuation providedby the device when the adjustable member is in contact with themembrane. This is because when the adjustable member is in contact withthe membrane, the tension in the membrane is increased and therefore theattenuation of sounds by the membrane increases. The measures providedin accordance with the invention however mean that there is much less ofa sharp change in performance between non-contact and contact asdiscussed previously than there would otherwise have been.

In a set of embodiments, in accordance with either the first or secondaspect of the invention, the adjustable member has a plurality ofpositions wherein the adjustable member is in contact with the membrane.The plurality of positions of the adjustable member correspond to aplurality of magnitudes of force applied to the adjustable membrane, andtherefore a plurality of tensions in the adjustable membrane. Forexample, as the adjustable member is moved further towards the membrane,the adjustable member exerts a greater force on the membrane andtherefore the tension of the membrane is increased. The attenuation ofthe membrane is therefore greater and a higher level of attenuation isprovided by the device.

As will be appreciated, it is desirable for the device to furthercomprise a mechanism for moving the adjustable member from one positionto another. The position of the adjustable member may be adjusted in anysuitable and desirable manner. In a set of embodiments, the devicefurther comprises an adjustment arrangement for adjusting the positionof the adjustable member. Preferably, the adjustment arrangementcomprises an actuator for adjusting the position of the adjustablemember.

In a set of embodiments of either the first or the second aspect of theinvention, the actuator comprises an electric motor. The use of anelectric motor may be advantageously mean that the device canautomatically adjust the position of the adjustable member withoutrequiring a physical input from a user. For example, the use of anelectric motor may mean that the device can automatically adjust theattenuation of the sounds in loud environment, without requiring theuser to take any action, thus protecting the user.

In another set of embodiments of either the first or the second aspectof the invention, the actuator comprises a user operable member. Theuser operable member may, for example, comprise a rotatable knobarranged to adjust the position of the adjustable member. A useroperable member may advantageously simplify the device and reduce itscost. Through the use of a user operable member, it may be possible toachieve a device which does not comprise any electrical/electroniccomponents, thereby potentially providing a device which does notrequire electrical power. Achieving a device which does not requireelectrical power may mean that the device is more frequently used asusers do not have to concern themselves with ensuring that the devicehas enough battery power for operation. This may help to improvecompliance with, for example, industry requirements to use hearingprotection.

Preferably, the user operable member is connected to the adjustablemember. In a set of embodiments, the user operate member is arranged torotate the adjustable member relative to its central axis.

The device may further include an arrangement for converting rotationalmovement of the user operable member (relative to the central axis) toaxial movement of the adjustable member. In a set of embodiments, theuser operable member comprises a threaded portion and the body comprisescorresponding threaded portion. In such a set of embodiments, thethreaded portion of the user operable member is arranged to engage witha threaded portion of the body. Therefore, rotation of the user operablemember causes a rotation of the adjustable member which results inlinear movement of the adjustable member (i.e. to move the adjustablemember towards or away from the membrane).

As will be appreciated by those skilled in the art, the pitch of thethreaded portions may be chosen to allow for highly controlled movementof the adjustable member. Such control may be required to preciselycontrol the tension of the membrane and therefore the level ofattenuation provided by the device.

More generally, the adjustable member or user operable member may engagethe body by means of one or more inclined surfaces so as to convertrotational movement of the user operable member and/or adjustable memberto axial movement of the adjustable member.

In a set of embodiments, the adjustable member is arranged such that itcan be held stable in a plurality of different positions. This may beachieved due to the presence of, for example, static friction.Alternatively, the device may comprise different means for holding theadjustable member stable. For example, the adjustable member and/or bodymay comprise a series of recesses and/or protrusions acting therebetweento hold the adjustable member stable when the recesses and protrusionsare in engagement with one another. The position of the adjustablemember could be thus adjustable continuously or incrementally.

In embodiments in which the actuator comprises a user operable member,different (e.g. rotational) positions of the user operable member maycorrespond to different stable positions of the adjustable member. Thesedifferent (e.g. rotational) positions of the user operable member may belabelled to demonstrate to the user the level of attenuation (e.g. high,medium, low) provided by the device when the user operable member is ina particular rotational position.

Especially in embodiments in which there is a rotational movement of theadjustable member (relative to the central axis), a torsional dragforces may be generated between the base of the adjustable member andthe membrane, which may inhibit smooth adjustments in the level ofattenuation provided by the device. In a set of embodiments, of eitherthe first or second aspects of the invention, the base of the adjustablemember which contacts the membrane in use comprises a low frictioncoating or layer. In a set of embodiments, the coating or layercomprises polytetrafluoroethylene (PTFE).The coating or layer may helpto reduce the aforementioned torsional drag forces between theadjustable member and the membrane. In embodiments wherein theadjustable member comprises a compressible portion, the compressibleportion may comprise the friction coating or layer, or a separate lowfiction coating or layer could be applied to the surface thereof whichcontacts the membrane. Alternatively, the adjustable member may beformed from a material with self-lubricating properties e.g.polyoxymethylene (POM) and/or the adjustable member may include frictionreducing additives e.g. polytetrafluoroethylene (PTFE).

The membrane may be in the form of a relatively thin sheet of materialon, or in, the device. Preferably thickness of the film is in the rangeof 1 to 20 μm. In an exemplary set of embodiments, the membrane is madefrom a plastic film e.g. polyethylene terephthalate (PET). Whilst thecorrugation can be provided in any suitable and desired way, in a set ofembodiments the corrugation is stamped into the membrane.

The membrane may be integral to the body of the device. For example, themembrane may be integrally moulded with the body, or the body may bemilled in order to form the membrane. However, the Applicant hasrecognised that integrally providing the membrane with the body may becomplicated to manufacture. In a set of embodiments in accordance withthe first or second aspect of the invention, the adjustable membrane isa separate component attached to the body. In a further set of suchembodiments, the body defines a rim to which the adjustable membrane isattached. This may allow for a simpler manufacture of the body.Additionally, it may allow the body and the adjustable membrane to bemanufactured from different materials which may be desirable in order toprovide a membrane with the required mechanical properties. For example,the adjustable membrane may be made from a plastic film, e.g.polyethylene terephthalate (PET) whilst the body may be made from e.g.,polycaprolactam (PA6) (e.g. the body may be formed from the samematerial as the adjustable member and the membrane formed from adifferent material).

In a set of embodiments, according to either the first or second aspectof the invention, the adjustable membrane is circular. This may allowfor a more uniform tensioning of the membrane, compared to membraneshaving alternative shapes, e.g. square shaped membranes. The diameter ofthe membrane may be in the range 4 mm to 20 mm (i.e. corresponding toradii in the range of 2 mm to 10 mm). In embodiments in which theadjustable membrane is circular and the body defines a rim to which themembrane is attached, it is preferable that the rim is also circular.

Preferably the membrane is essentially planar (e.g. in the planeperpendicular to the central axis of the adjustable member) and smooth,apart from any corrugations.

When the adjustable member is applying the least amount of force—whichmay be zero if the adjustable member has a position where it is not incontact with the membrane, the membrane can be considered to be under abase tension. The base tension may be provided solely by the attachmentof the membrane to the rim of the body. The base tension may also bedescribed as the minimum tension of the membrane. When the adjustablemember is moved into or further into contact with the membrane, thetension in the membrane is increased above the base tension. The basetension of the membrane may result in a relatively low level ofattenuation of sounds so that, for example, a user can still communicateorally with fellow workers.

In a set of embodiments, according to either the first or second aspectof the invention, the adjustable member is arranged to contact themembrane in the geometric centre of the membrane. In some embodiments,the geometric centre of the membrane is flat. In another set ofembodiments, the centre of the membrane is domed. For example, inembodiments wherein the membrane comprises a corrugation (e.g. a wave),the dome should preferably be smaller than the corrugation. The domeshape may help to control the contact between the adjustable member andthe membrane when the member first comes into contact with the membrane.

The membrane may be described as comprising part of an adjustableacousto-mechanical portion of the device. In a set of embodiments, thedevice comprises a further adjustable acousto-mechanical portioncomprising an adjustable channel forming at least part of the sound path(i.e. another part of the sound path to the membrane), and the devicefurther comprises an adjustment arrangement for adjusting the furtheracousto-mechanical portion. The further acousto-mechanical portion maybe arranged to vary the level and/or vary different qualities of theattenuation provided by the device.

In a set of embodiments, in accordance with either the first or thesecond aspects of the invention, the device comprises a first adjustableacousto-mechanical portion comprising an adjustable channel forming atleast part of the sound path and a second adjustable acousto-mechanicalportion arranged acoustically in series with the firstacousto-mechanical portion comprising the membrane as describedhereinabove, and an adjustment arrangement comprising said adjustablemember for simultaneously adjusting the first and the secondacousto-mechanical portions to alter the acoustic response of the atleast one sound path.

In this set of embodiments, the tensioned membrane constitutes anadjustable member forming part of a second adjustable acousto-mechanicalportion. The Applicant has recognised that with the arrangement of theadjustable channel and the adjustable membrane, it is possible toachieve an acoustic response of the sound path which does notsignificantly reduce the quality of the sound passing through the soundpath whilst maintaining the ability to control the sound, e.g. byattenuating the sound. This is because it allows the changes in thechannel and membrane as they are adjusted to complement one another tomaintain a favourable acoustic response.

As will be understood by those skilled in the art, the acoustic responseof the sound path should be understood to be how the sound path affectsthe sound which passes through it. The acoustic response of the soundpath may change the frequency, amplitude and/or phase of the soundpassing through it and thus ultimately change the sound heard by a userof the device.

The adjustment arrangement may comprise any suitable arrangement foradjusting the first adjustable acousto-mechanical portion and the secondadjustable acousto-mechanical portion. In a set of embodiments theadjustment arrangement, comprises a first actuator for adjusting thefirst adjustable acousto-mechanical portion and a second actuator foradjusting the adjustable member (i.e. that which is arranged to bearagainst the membrane). The first and second actuators may, for example,be connected to a single controller capable of simultaneouslycontrolling each of the first and second adjustable acousto-mechanicalportions. Such an arrangement may be particularly advantageous when thefirst and second adjustable acousto-mechanical portions requireadjustment by differing amounts, e.g. due to the need to achieve aparticular acoustic response. Having separate actuators for eachadjustable acousto-mechanical portion may make it possible to adjust oneof the portions through a greater proportion of its physical range ofmovement than the other if required in a particular instance to achievea desired response.

In an alternative set of embodiments in accordance with either aspect ofthe invention, the adjustment arrangement comprises a common actuatorarranged to adjust both the first and second acousto-mechanical portionssimultaneously. The Applicant has found that the use of such a commonactuator may be advantageous, for example in embodiments wherein thedevice is electrically powered as it may reduce the amount of powerrequired to adjust the first and second adjustable acousto-mechanicalportions. It may also simplify the construction of the device. Of courseby suitable design of such an actuator, e.g. to include one or morelevers or members of differing stiffness, different amounts of movementmay be imparted to the respective acousto-mechanical portions for agiven input movement. The common actuator could act on, or be providedby, the above-mentioned adjustable member.

In a set of embodiments in accordance with either aspects of theinvention, the or each actuator comprises an electric motor. However, inanother set of embodiments, the or each actuator comprises a useroperable member arranged to operate at least part of the adjustmentarrangement.

The first adjustable acousto-mechanical portion comprising theadjustable channel may be adjusted in any appropriate manner in order toachieve the desired acoustic response. In a set of embodiments theadjustment arrangement is configured to adjust a length of theadjustable channel. The Applicant has recognised that adjusting thelength of the adjustable channel may in general alter the effect of thechannel. Additionally or alternatively, the adjustment arrangement isconfigured to adjust a width of the adjustable channel. The Applicanthas found that adjusting the width of the adjustable channel may serveto adjust the specific acoustic properties of the sound path. Forexample, decreasing the width of the channel will typically increase theeffective acoustic mass and the acoustic loss of the channel and viceversa. Conversely decreasing the length of the channel will typicallydecrease the effective acoustic mass and the acoustic loss of thechannel and vice versa. In other words the acoustic mass and losstypically have a positive relationship with the length of the channeland a negative relationship with the width of the channel. The termsacoustic mass and acoustic loss are well known to those skilled in theart but will be further explained later.

As will be appreciated, the length and width of the adjustable channelmay be adjusted independently of one another, or simultaneouslytogether. In a potentially overlapping set of embodiments, theadjustment arrangement is configured to adjust a shape of the adjustablechannel.

The adjustable channel may be defined by any suitable structure withinthe device. For example, the channel may simply comprise a cylindricalchannel extending through the body of the device. Adjustment of such achannel may, for example, comprise constricting and expanding the bodyso as to decrease/increase the size of the channel. In a set ofembodiments in accordance with either the first or second aspect of theinvention, however, the channel is defined by a space between a wall ofa cavity within the body and a piston arranged in the cavity, whereinadjustment of the channel is achieved by moving the piston relative tothe cavity.

The piston may act on, or preferably be provided by, the adjustablemember of either of the first or second aspects of the invention.

The cross-section of the channel will depend on the shape of the wall ofthe cavity and the outer profile of the piston. The piston may have acomplementary sectional shape to the wall of the cavity, e.g. if thewall has a circular sectional shape, the piston may have a circularsectional shape. In such an example, the channel defined between thewall and piston would effectively be an elongate annular channel. TheApplicant has recognised that the arrangement of a piston in the cavityprovides for a relative simple means to adjust the length and/orcross-sectional area and/or shape of the channel.

The piston may be arranged in the device in any suitable manner suchthat it can be moved relative to the cavity. For example, the piston maybe a part of, or attached to, a linear actuator capable of moving thepiston into, and out of, the cavity. In a set of embodiments, the pistonis arranged to move axially within the cavity and the device comprisesat least one resilient member arranged to bias the piston into or out ofthe cavity, wherein the adjustment arrangement comprises an actuationmember arranged to drive the piston against the resilient bias axiallyout of or into the cavity respectively. The actuation member may bedriven by an electric motor or a user operable member. The Applicant hasrecognised that the provision of a resilient member arranged in themanner according to the above set of embodiments means that theactuation member needs only to be able to drive movement in one axialdirection as the resilient member is arranged to drive movement of thepiston in the other axial direction. This may simplify the manufactureand construction of the device. In a set of embodiments, the actuationmember is arranged to rotate relative to the piston, and the devicefurther comprises an arrangement for converting rotational movement ofthe actuation member into axial movement of the piston. The resilientmember may be integrally provided with the piston. In a further set ofembodiments, a plurality of resilient members is provided. In anotherset of embodiments, the at least one resilient member is in the form ofa resilient arm extending between the piston and the body.

Of course, in addition or alternatively, the piston may be arranged tomove in other directions other than just axially. For example, thepiston may be moved at a non-zero angle to the axis, be translated fromside-to-side within the cavity or even twisted, in order to adjust thechannel so as to achieve a desired acoustic response.

The Applicant has recognised that in order to appropriately control theacoustic response of the sound path, it may be necessary to adjust thelength and cross-section of the adjustable channel simultaneously. In afurther set of embodiments in accordance with either aspect of theinvention, the cavity and the piston each have a frusto-conical shapesuch that the adjustable channel has the form of a frusto-conical shell.The Applicant has recognised that in such an arrangement axial movementof the piston within the cavity may simultaneously adjust both thelength of the channel and the width. It may also mean that a relativelylarge axial movement can be converted into a relatively small change inwidth. This may help to simplify the manufacture and construction of thedevice and to provide fine control over the width. The piston and cavityare preferably shaped so that the channel remains of uniform shapethroughout the travel of the piston but this is not essential.

Some preferred embodiments of the present invention will now bedescribed, by way of example only, and with reference to theaccompanying drawings, in which:

FIG. 1 a shows an isometric view of part of a hearing protection adevice in accordance with an embodiment of the present invention;

FIG. 1 b shows an alternate isometric view of the device seen in FIG. 1a;

FIG. 2 a shows an exploded view of parts of the device seen in FIG. 1 a;

FIG. 2 b shows an isolated isometric view of the body of the device seenin FIG. 1 a;

FIG. 3 shows the piston, user operable member and the membrane of thedevice seen in FIG. 1 a;

FIG. 4 shows an isolated isometric view of the adjustable member anduser operable member of the device seen in FIG. 1 a;

FIG. 5 shows a cross-sectional view through certain components of thedevice in a first configuration;

FIG. 6 shows a cross-sectional view through certain components of thedevice in a second configuration;

FIG. 7 shows a cross-sectional view through certain components of thedevice in a third configuration;

FIG. 8 shows another cross-sectional view through the device seen inFIG. 1 a in the first configuration;

FIG. 9 shows another cross-sectional view through the device seen inFIG. 1 a in the second configuration;

FIG. 10 shows another cross-sectional view through the device seen inFIG. 1 a in the third configuration;

FIGS. 11 a and 11 b are illustrations demonstrating how the dimensionsof a channel can be adjusted using an adjustable member;

FIG. 12 shows a plan view of a membrane in accordance with anotherembodiment of the present invention;

FIG. 13 shows an alternative isometric view of the membrane seen in FIG.12 ;

FIGS. 14 a and 14 b show plan views of membranes in accordance withfurther embodiments of the present invention; and

FIGS. 15 a-15 c show views of membranes in accordance with furtherembodiments of the present invention.

FIG. 1 a shows an isometric view of part of a hearing protection devicein accordance with an embodiment of the present invention. The device isdesigned for insertion into a human inner ear canal and in practicewould be fitted with a flexible ‘cone’ or a bespoke moulded insert tofacilitate this. As can be seen in FIG. 1 a, the device 1 includes abody 2 upon which a user operable member in the form of a handle member4 is located.

The device 1 further comprises a resiliently compressible member 14,which is located upon the handle member 4. The compressible member 14 isarranged to apply a force to the various components of the device 1 tomaintain the handle member 4 and the body 2 in position with respect toeach other e.g. when the handle member 4 is rotated. The compressiblemember 14 can be compressed by varying degrees.

FIG. 1 b shows an isometric view of the underside of the device shown inFIG. 1 a. Arranged towards the base of the device 1 is a circularmembrane 10. Whilst the membrane shown in FIG. 1 b is transparent, thisis not essential and the membrane could be opaque. The membrane isattached to the body 2 of the device, in particular bonded to a circularrim 22 (as can be seen in more clearly in FIG. 2 a ) formed within thebody 2 of the device 1.

As can be seen from FIG. 1 b, the handle member 4 is integrally formedwith a tapering adjustable member in the form of a piston 6 whichcomprises a compressible portion 8 at its distal end in the form of adisc of compressible material such as a closed cell foam orthermoplastic elastomer. As will be explained later, the compressibleportion 8 is arranged to contact the membrane 10 to increase the tensionof the membrane 10. The piston 6 is attached to the handle member 4 bymeans of three spokes 26 which extended from the circumference of thehandle member 4 to the piston 6 so that movement of the handle member 4results in the movement (e.g. adjustments) of the piston 6. The movementof the piston 6 changes the degree of compression of the compressiblemember 14.

The handle member 4 can be rotated with respect to the body 2 of thedevice 1. In particular, the handle member 4 may be moved by a user to avariety of different rotation positions with respect to the body 2 ofthe device. When the handle member 4 is rotated, the piston 6 alsorotates. The conversion of rotational movements of the handle member 4to axial (linear) movement of the piston 6 will be discussed in moredetail in relation to FIGS. 5-10 . As will also be discussed in furtherdetail in relation to FIGS. 5-10 , different rotational positions of thehandle member 4 correspond to different attenuations provided by thedevice 1.

The membrane 10 further comprises a corrugation 12. The corrugation 12can be seen more clearly in FIG. 3 which shows an isometric view of theisolated handle member 4 and membrane 10 of the device shown in FIG. 1 .The corrugation 12 is a circular ridge centred on the geometric centreof the membrane 10 which extends above the plane of the membrane 10.This arrangement of the corrugation 12 means that the compressibleportion 8 does not come into contact with the corrugation 12 (e.g. thecompressible portion 8 contacts the membrane 10 in the geometric centreof the membrane 10). The cross-section of the ridge resembles a bellcurve, which can be seen more clearly in the cross-sectional views seenin FIG. 5-7 .

FIG. 2 a shows an exploded view of the device seen in FIG. 1 . Startingfrom the top, FIG. 2 a shows the compressible member 14 and the handlemember 4. The handle member 4 is essentially circular with a protrudinggrip portion. As previously discussed, the handle member 4 is integrallyformed with a piston 6. The piston 6 extends below the handle member 4and is concentric with it. FIG. 2 a also shows the tensioned membrane 10which comprises the aforementioned corrugation 12.

In FIG. 2 a, the rim 22 which forms part of the body 2 of the device 1can be seen. The membrane 10 is secured in position on the rim 22 by astabilising ring 16 and two adhesive tape rings 18, 19. The stabilisingring 16 and tapes 18, 19 have the same shape as the rim 22. When thedevice is constructed, the first tape 18 is positioned between the rim22 and the membrane 10. The second tape 19 is positioned between themembrane 10 and the stabilising ring 16. The stabilising ring 16 holdsthe membrane 10 in position during formation of the corrugation 12 inthe production process.

In the embodiment of the device shown in FIG. 2 b, the body 2 includesthree inclined cam surfaces 20, located on the uppermost surface of thebody 2. FIG. 4 shows an isometric view of the underside of the handlemember 4 and piston 6 in isolation. In this isolated view, the undersideof each of the spokes 26 of the handle member 4 can be seen to comprisea ridge 28 on its underside. When the device is assembled (e.g. as seenin FIGS. 1 a and 1 b ), each of the ridges 28 contacts a correspondinginclined cam surface 20 of the body 2 of the device 1.

Operation of the device will now be described with reference to FIGS. 5to 10 .

FIGS. 5-7 are illustrations demonstrating how the position of the piston6 can be used to adjust the tension of the membrane 10. For clarity,these illustrations show a cross-section through the handle member 4,the piston 6 and the membrane 10 in isolation from the other componentsof the device 1.

FIGS. 8-10 are illustrations demonstrating how the position of thepiston 6 can also be used to adjust the dimensions of a channel 24. Inthe embodiments shown in the FIGS. 5-10 , the piston 6 simultaneouslyadjusts the dimensions of the channel 24 and the tension of the membrane10.

When the handle member is in the first position shown in FIGS. 5 and 8 ,each ridge 28 contacts the corresponding inclined cam surface 20 at itshighest point. As a result of this, the piston 6 is not in contact withthe membrane 10. In this position, the piston 6 applies zero force tothe membrane 10. The tension in the membrane 10 is therefore the basetension (i.e. the minimum tension in the membrane). This base tension isprovided by the attachment of the membrane 10 to the rim 22 of the body2. The compressible member 14 is in a compressed state, which biases themovement of the piston 6 axially downwards towards the membrane 10.

As shown in FIG. 8 , when the handle member 4 is in the first position,a channel 24 is formed between the wall 30 of the cavity formed by thebody 2 of the device 1 and the piston 6. The arrows illustrate how soundpropagates through the device along a sound path which includes thechannel 24.

The handle member 4 may then be rotated by the user to a secondposition, as shown in FIGS. 6 and 9 . In the second position of thehandle member 4, each ridge 28 contacts the corresponding inclined camsurface 20 at the middle point of the inclined cam surface 20. When thehandle member is rotated from the first position to the second position,each ridge moves down the corresponding inclined cam surface 20. Thewhole of the handle member 4 moves axially downwards towards the body 2of the device 1 and therefore the piston 6 is moved axially downwardstowards the membrane 10. Together, the inclined cam surfaces 20 andridges 28 on the spokes 26 convert the rotational movement of the handlemember 4 to axial movement of the piston 6.

In the second position shown in FIG. 6 , the piston 6 is in a positionin which the piston 6 just contacts the surface of the membrane 10. Inthe absence of a corrugation, when the piston 6 contacts the membrane10, the primary vibration mode of the membrane 10 is disabled and thecentre of the membrane 10 can be considered to be held stationary.Therefore, only higher order harmonic vibration modes of the membrane 10are enabled, resulting in an abrupt increase in the attenuation.Including the corrugation 12 in the membrane reduces this effect.Therefore, the level of attenuation is more smoothly increased when thepiston just contacts such a membrane 10 which includes a corrugation 12.

As the piston 6 comprises a compressible portion 8, some of the forcewhich would have otherwise been exerted on the membrane 10 (e.g.compared with a rigid piston with no compressible portion) acts tocompress the compressible portion 8. This results in a smaller forcebeing exerted on the membrane 10 by the piston 6 and therefore a smallerincrease in tension of the membrane 10. The level of attenuation istherefore increased more gradually when the piston 6 is moved to justcontact the surface of the membrane 10 (compared with a rigid member).

In the second position as shown in FIG. 9 , the piston 6 is positionedso as to decrease the dimensions (e.g. the width and/or the length) ofthe channel 24 compared with the first position. As will be describedlater, decreasing the width of the channel 24 increases the effectiveacoustic mass and the acoustic loss of the channel 24. Decreasing thelength of the channel 24 decreases the effective acoustic mass and theacoustic loss of the channel 24. This, together with the altered tensionof the membrane 10, changes the overall response of the device 1.

The handle member 4 may be further rotated by the user to a thirdposition, as shown in FIGS. 7 and 10 . In the third position of thehandle member 4, each ridge 28 contacts the corresponding inclined camsurface 20 at the lowest point. This corresponds to limit of rotationalmovement of the handle member 4 (in this direction) and therefore thelimit of axial movement of the piston (towards the membrane).

In the third position shown in FIG. 7 , the piston is in contact withthe surface of the membrane, and exerts a larger (e.g. a maximum) forceon the membrane (e.g. than was exerted when the handle member 4 was in asecond position and the piston 6 just contacted the surface of themembrane 10). The force exerted by the piston 6 also causes a morepronounced deformation of the rest of the membrane 10. As can be seen inFIG. 7 , in this position the membrane 10 is deformed from essentiallyplanar to concave or frusto-conical as a result of the force exerted onthe membrane 10 by the piston 6. In this configuration, the tension ofthe membrane 10 is increased further above the base tension (e.g. thetension is increased to its maximum). This results in the attenuationprovided by the device 1 being greater (i.e. than the attenuationprovided by the device 1 when the handle member is in the first orsecond positions shown in FIGS. 5 and 6 respectively).

In the third position shown in FIG. 10 , the piston 6 contacts the wallsof the cavity in body 2 of the device 1. Therefore, there is a minimalchannel width (e.g. no channel) between the walls of the cavity of thebody and the piston 6. In this arrangement, the minimal channel widthtogether with the high tension of the membrane, both results in thedevice providing its maximum attenuation. The compressible member 14 isdecompressed.

In embodiments in which the attenuation provided by the device is at aminimum, when the handle member is in the first position, the channel 24may be described as ‘open’ and the device 1 may be described as being inan ‘open’ configuration. In embodiments in which the attenuationprovided by the device is at a maximum when the handle member is in thethird position, the channel 24 may be described as ‘closed’ and thedevice 1 may be described as being in a ‘closed’ configuration.

Of course, as will be appreciated by those skilled in the art, thehandle member 4 may be moved to any intermediate position between thefirst, second and third positions seen in FIGS. 5-7 and 8-10 , in orderto achieve a desired acoustic response of the sound path.

FIGS. 11 a-11 b are illustrations demonstrating how the position of apiston, or in the device shown in FIG. 1 , the piston 6 in the cavity ofthe body 2 can be used to adjust the channel 24. When the piston is inthe position seen in FIG. 11 a, the channel 24 has a length shown byarrow 34 and a width shown by arrow 32. As demonstrated by FIG. 11 b,when the piston 4 is moved axially into the cavity, the length of thechannel shown by arrow 34 and the width of the channel shown by arrow 32are both changed. Accordingly, changing the axial position of the piston4 relative to the cavity of the body 2 will adjust the channel 24. Aswill be appreciated by those skilled in the art, adjusting thedimensions of the channel 24 will serve to alter the acoustic responseof the channel 24 and thus alter the acoustic response of the soundpath. More specifically the width, d, of the channel is related to theacoustic loss and the acoustic mass of the channel. Under the electricalcircuit analogy for analysis of acoustic systems which will be familiarto those skilled in the art, the acoustic loss is equivalent to aresistance R and has an inverse cube relationship to the channel widthas shown below:

$\begin{matrix}{R = {k_{1}\frac{1}{d^{3}}}} & {\left( {{Eq}1} \right).}\end{matrix}$

where k₁ is a constant representing parameters assumed to remainconstant such as air density and dimensions.

Under the same electrical analogy, the acoustic mass is equivalent to aninductance, L and has an inverse relationship to the channel width, d:

$\begin{matrix}{L = {k_{2}\frac{1}{d}}} & {\left( {{Eq}2} \right).}\end{matrix}$

where k₂ is a constant representing parameters assumed to remainconstant such as air density and dimensions.

The resistance R and the inductance L are both directly proportional tothe length of the channel.

Following the aforementioned electrical analogy, when a membrane istensioned the acoustic capacitance will decrease. As this capacitance isin series with the acoustic capacity of the ear canal, but also theresistance and inductance of the transmission path through the channel,this will cause the attenuation to increase, but will also change thefrequency response. In accordance with the invention the changes inacoustic resistance, inductance and capacitance can be tuned so that thechange in frequency response matches the natural frequency response ofthe human ear so that the frequency spectrum perceived by a user isessentially flat.

In the device 1, shown in the Figures when the user rotates the handlemember 4, the dimension of the channel 24 and the tension of themembrane 10 are simultaneously adjusted. The simultaneous changes in thedimensions of the channel 24 and the tension of the membrane 10complement one another, and therefore it is possible to achieve anacoustic response of the sound path which does not significantly reducethe quality of the sound passing through the sound path whilstmaintaining the ability to control the sound e.g. by attenuating thesound.

FIG. 12 and FIG. 13 show plan and isometric views respectively of amembrane 100 in accordance with another embodiment the invention. Thismembrane can be used in any of the hearing protection device embodimentsdescribed above or indeed any other such embodiments. The membrane 100is formed on and supported by a brass ring 101 which extend around thecircumference of the membrane. As can be seen more clearly from FIG. 13, the membrane 100 comprises three waves 102, 104, 106. The innermostwave 102 and the outermost wave 106 are ‘positive’ waves, extendingabove higher than the centre of the membrane 118. The intervening wave104 is a ‘negative’ wave extending lower than height of the centre ofthe membrane 118.

The membrane 100 also includes three sets of perturbations 112, 114, 116corresponding to waves 102, 104, 106. The perturbations 112, 114, 116reduce stress in the waves 102, 104, 106, allowing the system to vibratemore freely.

The perturbations 112, 114, 116 are spaced evenly around thecircumference of their corresponding wave 102, 104, 106. As the samenumber of perturbations 112, 114, 116 are present on each wave, thedensity of the perturbations on the innermost wave 102 is the greatest(i.e. their relative spacing is the smallest) whilst the density of theperturbations on the outermost wave 106 is the lowest.

The perturbations 112, 114, 116 extend towards the tangential plane ofthe centre of the membrane 118. When viewed from the angle shown in FIG.13 , perturbations 112, 116 on the positive waves 102, 104 appear asindentations, whereas perturbations 114 on the negative wave 104 appearas projections. The perturbations 112, 114, 116 are non-radial,diverging from the radius of the membrane 100 by the same angle.

The central portion 118 of the membrane 100 is slightly domed (notvisible in Figures). When the membrane is implemented in the device 1shown in FIG. 1 , the piston would be arranged to contact the membraneat its centre 118.

The particular arrangement of waves 102, 104, 106 and perturbations 112,114, 116 shown in FIGS. 12 and 13 provide a membrane 100 which canvibrate freely when the piston is not touching the membrane and becomeincreasingly stiff when the tension of the membrane is increased by thepiston. The arrangement also minimises creep in the membrane 100 at hightensions, which can result in the changes in behaviour of the membraneto vibrations over time.

FIGS. 14 a and 14 b show plan views of membranes in accordance withfurther embodiments of the present invention. Similar to the membrane100, shown in FIGS. 12 and 13 , the membranes 200, 300 shown in FIGS. 14a and 14 b include three waves and perturbations on each wave.

The perturbations in FIGS. 14 a and 14 b differs from that seen in FIGS.12 and 13 . Membranes 200, 300 shown in FIGS. 14 a and 14 b have fewerperturbations on the innermost wave 202, 302. The innermost wave 202,302 comprises ten perturbations, whereas the intervening wave 204, 304and the outermost wave 206, 306 comprise twenty perturbations.

The membrane 400, shown in FIG. 14 b, includes an additional variationin the arrangement of the perturbations. The perturbations on the middlewave 304 are angled in an opposite non-radial direction compared withthe perturbations on the innermost wave 302 and the outermost wave 304.

FIGS. 15 a to 15 c show views of membranes in accordance with otherpossible embodiments of the present invention. For example the centralportions 418, 518, 618 of the membranes 400, 500, 600 shown in FIGS. 15a to 15 c have a smaller diameters compared with those seen in FIGS. 12to 14 b.

The membrane 400 shown in FIG. 15 a has no perturbations on the middlewave 404. The innermost wave on the membrane 400 has fifteenperturbations, whereas the outermost wave has twenty five perturbations.

The membrane 500 shown in FIG. 15 b has perturbations on all threewaves. The innermost wave on the membrane 500 has fifteen perturbations,whereas the intervening and outermost wave have twenty fiveperturbations.

The membrane 600 shown in FIG. 15 c has a similar arrangement ofperturbations to that shown in FIG. 14 a.

Of course there are many other variants of waves and perturbationspossible in accordance with the invention.

1. A device, for insertion into an ear canal of a mammalian subject,comprising: a body, having a sound path extending therethough; atensioned membrane in the sound path comprising at least onecorrugation; and an adjustable member arranged to bear against themembrane to adjust the tension of the membrane and thereby to alter anacoustic response of the sound path.
 2. A device as claimed in claim 1,wherein the corrugation is arranged on the membrane such that theadjustable member does not contact the corrugation.
 3. A device asclaimed in claim 1, wherein the adjustable member is arranged to contactthe membrane in a geometric centre of the membrane.
 4. The device asclaimed in claim 1, wherein the adjustable membrane comprises aplurality of waves, each wave comprising a corrugation in the form of acircular ridge or indentation.
 5. The device as claimed in claim 4,wherein the waves comprise alternating circular indentations and ridges.6. The device as claimed in claim 4, wherein the membrane comprisesthree waves.
 7. The device as claimed in claim 4, wherein each wavecomprises a plurality of circumferentially spaced perturbations.
 8. Thedevice as claimed in claim 7, wherein the perturbations extend in anopposite direction to the corresponding wave on which they are formed.9. The device as claimed in claim 7, wherein the perturbations arenon-radial.
 10. The device as claimed in claim 7, wherein each wavecomprises the same number of perturbations.
 11. The device as claimed inclaim 1, wherein the adjustable member comprises a compressible portion.12. A device, for insertion into an ear canal of a mammalian subject,comprising: a body, having a sound path extending therethough; atensioned membrane in the sound path; and an adjustable membercomprising a compressible portion and arranged to bear against theadjustable membrane to adjust the tension of the membrane and thereby toalter an acoustic response of the sound path.
 13. The device as claimedin claim 11, wherein the compressible portion of the adjustable memberis arranged to contact the surface of the membrane.
 14. The device asclaimed in claim 11, wherein the compressible portion is formed from alayer of inherently compressible material.
 15. The device as claimed inclaim 1, wherein the adjustable member has a position wherein theadjustable member is not in contact with the membrane.
 16. The device asclaimed in claim 1, wherein the adjustable member has a plurality ofpositions wherein the adjustable member is in contact with the membrane.17. The device as claimed in claim 1, further comprising a user operablemember arranged to rotate the adjustable member relative to a centralaxis thereof for adjusting a position of the adjustable member.
 18. Thedevice as claimed in claim 17, further comprising an arrangement forconverting rotational movement of the user operable member to axialmovement of the adjustable member.
 19. The device as claimed in claim 1,wherein the adjustable member comprises a base which contacts themembrane in use and which comprises a low friction coating or layer. 20.The device as claimed in claim 1, wherein a or the centre of themembrane is domed.
 21. The device as claimed in claim 1, wherein themembrane comprises part of an adjustable acousto-mechanical portion ofthe device and wherein the device comprises a further adjustableacousto-mechanical portion comprising an adjustable channel forming atleast part of the sound path, and the device further comprises anadjustment arrangement for adjusting the further acousto-mechanicalportion.
 22. The device as claimed in claim 1, wherein the devicecomprises a first adjustable acousto-mechanical portion comprising anadjustable channel forming at least part of the sound path and a secondadjustable acousto-mechanical portion arranged acoustically in serieswith the first acousto-mechanical portion comprising the membrane, andan adjustment arrangement comprising said adjustable member forsimultaneously adjusting the first and the second acousto-mechanicalportions to alter the acoustic response of the at least one sound path.23. The device as claimed in claim 21, wherein the adjustmentarrangement comprises a common actuator arranged to adjust bothacousto-mechanical portions simultaneously.
 24. The device as claimed inclaim 21, wherein the adjustment arrangement is configured to adjust alength and/or width of the adjustable channel
 25. The device as claimedin claim 21, wherein the channel is defined by a space between a wall ofa cavity within the body and a piston arranged in the cavity, whereinadjustment of the channel is achieved by moving the piston relative tothe cavity.
 26. The device as claimed in claim 25, wherein the piston isprovided by the adjustable member.
 27. The device as claimed in claim25, wherein the piston is arranged to move axially within the cavity andthe device comprises at least one resilient member arranged to bias thepiston into or out of the cavity, wherein the adjustment arrangementcomprises an actuation member arranged to drive the piston against theresilient bias axially out of or into the cavity respectively.
 28. Thedevice as claimed in claim 25, wherein the cavity and the piston eachhave a frusto-conical shape such that the adjustable channel has theform of a frusto-conical shell.